Weird clicking sound noise on the back if my imac

I have a 2009 24" iMac with a 1Tb HD drive. It has been making a weird clicking sound for about an hour. Whenever I hear the sound the computer also starts to "think" as if it's trying to execute something processor intensive, when all I'm doing is writing this post.

You'd be more likely to hear this noise if the drive is put to sleep and has been idle for an extended period. Just to be sure, you should run check SMART status of the drive. Onyx, a free utility for OS X, will do this. A better tool though is SMART Utility. It will do a more thorough analysis of the SMART data and can uncover signs of impending drive failure far sooner than any other SMART tool can. It's not free, but does have a free trial so take advantage of it.